I purchased this as a planetary eyepiece for use with a 4 inch / 880mm refractor. The purchase was also based on very favorable experience with both the 6mm and 10mm Radians, and the expectations that they would all perform pretty much the same way.

Reality hasn't matched expectations in this case. The 4mm Radian seems to be very unforgiving in terms of off-axis eye placement. The kidney bean blackout effect is much worse in the 4mm. I find both the 6mm and 10mm Radians quite simple and a joy to use in comparisson to the 4mm.

Once you get past the eye placement / ease of use issue, this is a very fine eyepiece. Sharpness and contrast are similar to the other Radians. I write this review only to warn others that the 4mm behaves a little differently than some of it's longer focal length sibblings.

Sharp and contrasty eyepiece and a great planetary one. The eye relief is fantastic and allows a full vie with glasses on. Is it perfect? No, compared to the 3mm Radian, the 4mm is more prone to blackout and is harder to hold the view. Thus, it is an eyepiece that will give difficulty to beginners and young children. The "insta-adjust" eyepiece is a love it or hate it thing and I definetly hate it! There is a tiny amount of false colour at the field's edge. It is easily ignored but if you are very finnicky about such things, beware! Overall, it is an outstanding eyepiece with some drawbacks.

On my 8" f/5 Dob, this gives me a .8mm exit pupil and 250x: ideal for close-up viewsof the moon (esp when the seeing doesn't permit silly magnification games involvinga 2.5x Powermate and a Tak LE5;) and excellent for Mars. Impressive workmanship -I own several eps with useless filter threads but my R4 will accept and hold each ofmy filters neatly. I'm not overly fond of the InstAdjust feature, but am getting usedto it with practice, as well as with eye positioning (I like the pupil guide disk).This is not an ep for casual viewing, it's for careful and deliberate savoring, likea good Bordeaux...

Used with a low-cost SW150 refractor, it gives tremendous views of Saturn. The images are contrasty and sharp to the edge. On a steady night (seeing=8+) you can see Cassini's division all the way around quite easily. The Crepe ring is easy to see also. You can see shaded detail in the B ring as it gets closer to the Crepe. The Encke minima is also visible in the A-ring in moments of best steadiness. You can see up to two bands on the disk.

There is a little color at the very edges, but you really have to look for it. Otherwise, with its wide field and excellent eyerelief, this is a great eyepiece to use. Hard to evaluate its use with a Barlow since 300X is about the upper limit on a SW150.

This eyepiece is an Abbe Ortho on steroids. Fantastic brightness, sharpness and contrast, with a wide field AND incredible (at 4mm fl) 20mm of eye relief. I sold a 4mm UO Orthoscopic to buy this. (My astigmatism is now too strong for short eye relief eyepieces.)The “instajust” shade takes getting used to, but I’ve begun to like it very much. Expensive, but like my 10mm Radian, worth it.

Sharpness and brightness are excellent. This is an incredible eyepiece. I prefer it over the nagler 3-6 zoom --- the 10 degree field bonus makes me enjoy the view more (don't get me wrong, I _love_ the zoom). With any other eyepiece type, I'd have to say that my preferred planetary magnification w/ TV85 is 120x. However, the 4mm Radian beats the latter by a country mile. Simply outstanding...

In using this eyepiece in my 6" refractor I find the views of Jupiter and Saturn to be outstanding. Great eye relief and sharp detail all the way out to the edge of view. Viewing the moon at 300x is nothing short of incredible. Anyone who has not looked at the moon thru a 4mm Radian in a good refractor should treat themselves the next time the opportunity arises. There is nothing like a decent telescope and a great eyepiece together. TeleVues are not cheap but when you consider what you get for what you pay, it is well worth the investment.

Very sharp eyepiece, although I think the Naglers are still better. Almost as good as the Vixen LV for "starparty/newbie" use in terms of ease of viewing, but superior to the LV on sharpenss and ghosting. Much bigger in size than it appears, equally as big in diameter as a 19 Panoptic, but much longer. Clickable eyeguard takes some getting used to, but is easy to adjust for different users.

I was very worried receiving this EP when I saw how huge it was and how small the bottom lens is. I figured "oh boy, this might end up being a wash out mistake of an EP when I could have put a downpayment on a Ferrari!" But...when placed into my TeleVue 76 APO, it was a whole different story! Excellent contrast and sharpness. Saturn has excellent razor sharp detail with a fine hairline dark Cassini Division and Jupiter shows fine detail on the cloudbands. This is an excellent EP with fantastic eye relief. The pupil guide is a Godsend for using at starparties due to blackout if your eye is not trained to using a EP like this. The click stop really works great and you can actually see the FOV expand open with each click. The views are rich and sharp, but a little darker than a Plossl or Ortho, but then again, the sharpness given by this Radian cannot be challenged by a Plossl or Ortho. On the Moon, the high amount of lenses used for this EP work great, with the dimmer view of the Radian enhancing contrast on the Moon. If you have the money...go for it!...it is worth it